This invention relates to a slew-rate controller of power transistors in a half-bridge output stage employing a complemetary transistor pair.
When driving inductive loads in a switching mode, abrupt changes of current in the windings may cause electromagnetic emissions, besides causing torque discontinuities in electric motors that cause noisy operation.
A typical situation is illustrated in FIG. 1, which shows three separate half-bridge output stages, each driving one winding (L1, L2, L3) of a three-phase brushless DC motor. If, for example, it is necessary to switch the current from the power transistor M1 to the power transistor M2, an abrupt turn-off of M1 makes the inductance of the winding L1 discharge on the supply rail through the diode D4. This causes a drop in the current through the sensing resistance Rs of the control system, and a consequent change in the current passing through the excitation winding of the motor. In fact, the current regulating circuit will respond to keep the voltage across the sensing resistance Rs constant, by changing the bias conditions of the output transistors thus altering the current in the motor windings. This causes an undesirable variation of the torque.
However, the injection of the discharge current of the winding's inductance on the supply rail may cause disturbances on the line. It is common knowledge that these problems may be considerably reduced by controlling the turn-off time of the power transistor. In doing so, the following should be taken into account:
a) the turn-off time must be long enough to allow the inductance to fully discharge; but PA1 b) an excessively long turn-off delay would negatively affect speed performance, e.g. in driving a motor.
Commonly, a turn-off slew-rate control is individually implemented for each output power transistor of the stage.
On the other hand, depending on the characteristics of the fabrication process of the integrated circuit, complementary and structurally similar architectures are highly preferable due to the advantages of simpler design and layout and higher overall performance. Moreover, a half-bridge output stage architecture employing a :pair of push-pull connected complementary power transistors (e.g. PMOS and NMOS) offers significant advantages as compared to architectures employing transistors of only one type (which, in a bridge configuration, require driving circuits that are not mirror images of each other).
An innovative slew-rate controller has now been devised, and (as disclosed herein) has proven itself particularly effective in controlling the turn-off slew-rate of the output transistors of a half-bridge stage. This controller is both extremely simple to realize, and also cost-efficient. The novel controller of the invention employs a pair of push-pull connected, complementary, power transistors. A main advantage of the novel device of the invention is that a single capacitor is shared by two structurally similar integrating stages that are used to drive the two complementary output transistors, respectively, so as to control their slew-rates during a turn-off phase. A single integrating capacitor is switched onto one or the other stage by two switches, controlled in phase opposition to each other by a pair of control signals that are suitably out-of-phase with respect to the pair of logic signals that drive the two complementary output transistors, respectively.
Another important advantage of the device of the invention, when employed in the driving circuitry of a multi-phase brushless DC motor, is that it does not require a dedicated circuit for generating the control (timing) signals for the pair of switches used in the slew-rate controller circuit. In fact, a pair of driving signals for a different excitation phase of the motor can advantageously be used for that purpose, since they are already available within the motor drive system itself.